Almost two million years ago, a band of brave explorers left their families behind in their warm, tropical home and sought refuge in northern lands. Armed with sharp stone tools and their wits, they followed the coast as far north as they could, then began to veer east, settling on the sunny, fertile shores of an inland sea that today we call the Mediterranean. Their children spread further north and east, and a million years later they had established settlements along the coasts of today's Europe, England, and China.

A few hundred thousand years passed, when suddenly a new wave of immigrants emerged from Africa - the children of all the people our first adventurers left behind. They swarmed off the continent, following the route of their brethren. But what happened next? Did the new immigrants eradicate their strange cousins and colonize their lands? Settle down with them and have families? Or were they not strangers at all, but just far-flung satellites in the same family, who had kept in distant touch via trade routes for a million years?

Most of us are familiar with the basic outlines of the human evolutionary story. Our distant ancestors were a group of ape-like creatures who started walking upright millions of years ago in Africa, eventually developing bigger brains and scattering throughout the world to become modern humans of today. Now, advances in genetics have given us a sharper understanding of what happened in between the "scattering" and the "buying the latest iPad" chapters of the tale. The question is, which version of the story do you believe? It's one of the biggest questions in human evolution today. Here's what you need to know about it.

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Who are the heroes of our story?

Human evolution wasn't a simple linear progression from ape-like hominid, to the humans of today. Early humans moved through several stages of evolution over time, but they were also wanderers who moved through many spaces. As they spread out across the land from their origins in southern Africa, they separated into different bands but continued to evolve. Our story here is about what happened to us as we scattered across the globe, and there are four major players in this evolutionary drama.

About two million years ago there was an archaic human called Homo ergaster who lived in Africa. She used fairly sophisticated methods to create stone tools, and taught those methods to her children. At some point, probably about 1.8 million years ago, H. ergaster split into many different bands. Some wound up crossing out of Africa and into the Middle East, Asia and Europe. Others stayed behind.

This is where things begin to get interesting. (See map above.) The H. ergaster groups who headed out into Asia eventually developed their own culture and distinct skull structure. They were evolving in a very different environment from their cousins back in Africa, so their bodies changed and so did their toolsets. Most of what remains from this era is fragmentary at best - a few bits of human skeletons, and a lot of stone tools. So we can track how the tools change more easily than how our ancestors' bodies did. Based on a combination of these new tools and a few telltale skull shape differences, scientists have dubbed these people Homo erectus. Their culture and communities lasted for hundreds of thousands of years, and spread throughout China and down into Java.

At the same time, another group of H. ergaster was drifting into Europe, creating homes in what are now Italy, Spain, France, Germany, and England, among others. They evolved a thicker brow and more barrel-chested body. These are the early humans popularly called Neandertals. Anthroplogists call them Homo neanderthalensis.

Back in Africa, H. ergaster was busy too. She was establishing homebases all over the coasts of the continent, reaching from South Africa all the way up to Algeria and Morocco. And about 200 thousand years ago, H. ergaster's skeletal shape had become indistinguishable from those of modern humans. Homo sapiens had emerged. And now things get complicated.

The "African replacement" theory, sometimes called the "recent African origins" theory, holds that H. sapiens charged out Africa and crushed H. neanderthalensis and H. erectus under her feet. Basically H. sapiens replaced her distant cousins. This theory is simple, and the "mitochondrial Eve" discoveries of biochemist Rebecca Cann and colleagues support it with genetic evidence that shows all humans on Earth can trace their genetic ancestry to a H. sapiens woman from Africa.

But if you step back for a second and look at this theory from a historical perspective it starts to seem more unlikely. First of all, it assumes that H. sapiens treated her brethren as enemies, or as some anthropologists seem to suggest, she saw them as animals rather than members of her family. The question is: How likely is it that a group of tired H. sapiens wanderers, coming upon a community of H. erectus with tools and recognizably human faces, would attack them or ignore them as "animals"? Most likely they would trade with the locals, and possibly spend a while hanging out with them as they rested on their long journey.

And that's the kind of thinking that got the multi-regional theory started. Popularized by anthropologist Milford Wolpoff (see one of his papers on it here [PDF]), this theory fits with the same evidence that supports the African replacement theory - it's just a very different interpretation of the evidence. Wolpoff suggested that H. sapiens didn't sweep H. erectus and H. neanderthalensis away, but instead never really lost track of them in the first place.

Wolpoff's idea hinges on the very sensible notion that H. ergaster didn't leave Africa, but instead forged a pathway that many other archaic humans followed - in both directions. Just as humans had trade routes that linked far-flung lands in recorded history, our earliest ancestors probably had something similar. There is plenty of evidence that humans left many outposts along the route from Africa to Asia and Europe. Who is to say H. sapiens wasn't always intermingling and interbreeding with H. erectus and H. neanderthalensis? If Wolpoff and his colleagues are right - and evolutionary biologist John Hawks has presented compelling genetic evidence for this [PDF] - then H. sapiens probably didn't arise in Africa and colonize the rest of the world. Instead, she arose at roughly the same time throughout the world through this extended network.

The multi-regional theory does not suggest that two or three separate human lineages evolved in parallel, by the way. That's a common misinterpretation. It just suggests that there weren't two distinct waves of immigration like that map above suggests. Instead, immigration (and evolution of H. sapiens) started 1.8 million years ago and never stopped.

There is a kind of middle-of-the-road theory, too, which many dub the assimilation theory. Vinayak Eswaran and colleagues outline a theory like this in a recent paper, where they argue that genetic evidence suggests that there were two distinct waves of immigration out of Africa - the archaic human one, and the H. sapiens one. But as H. sapiens moved out into the world, she assimilated the local H. erectus and H. neanderthalensis peoples.

So basically, in the assimilation theory model H. sapiens didn't destroy her kindred, nor was she deeply interrelated with them as in the multi-regional theory. She met them as strangers, but forged alliances and formed families with them. Gradually, though, H. sapiens became the dominant culture.

Why do we know so little about this?

Anthropologists agree on most basic facts about where people migrated and when. How can we have three such divergent theories? The simple answer is that the evidence is scarce: Some stages in human evolution only appear in one or two bones.

Most of our information about where our ancestors lived come from finding tools because stone preserves better than bones do. As a result, scientists will often classify a discovery as belonging to H. erectus, for example, based on the kinds of tools they find and not on skeletal remains. So in a sense, our view of human history is based more on cultural artifacts than it is on biological ones.

Finding and dating these artifacts presents a number of problems, ranging from access (if there were H. ergaster cultural remains in Afghanistan, how would we go about excavating them?) to technological limits on dating (often we have to date the age of artifacts based on where they appear in sedimentary layers).

Until recently, the only way we could trace our routes out of Africa was by searching for fossils and artifacts. Anthropologists could track evolutionary and cultural changes by examining ancient remains, dating them with a variety of technologies, and extrapolating a route out of Africa based on those scarce findings.

Most anthropologists are comfortable admitting that we just don't know what happened when early humans left Africa, and are used to revising their theories when new evidence presents itself. Richard Klein's influential textbook The Human Career, which I highly recommend as a detailed primer on human evolution, is full of caveats about how many of these theories are under constant debate and revision.

Just this year, for example, anthropologist Simon Armitage argued in a paper that archaeological evidence suggests H. sapiens emerge from Africa as early as 200 thousand years ago, settling in the Middle East. This flies in the face of previous theories, which hold that H. sapiens didn't leave Africa until about 70 thousand years ago.

Genetic evidence

Today we are supplementing studies of human bones and artifacts with genetic studies. These studies rely on sampling DNA from representative people all over the world, and looking at how similar they are. If there was a recent wave of immigration from Africa, what you'd expect to see is people's DNA becoming more and more similar the further away from Africa they are. This is a result of what's called the "founder effect."

If you look at the illustration, you'll see a simple representation of the founder effect. As the original band splits into founder groups, their genetic diversity is lessened. As a result, when a population leaves an area you expect it to become less and less genetically diverse. And indeed, several studies have shown that humans all over the world are genetically very similar, with the most genetically diverse populations in Africa and India (the second place that H. sapiens settled).

A low genetic diversity among humans could suggest that the replacement theory is correct. As H. sapiens spread out of Africa, she replaced the archaic human populations and left only her own genetic traces behind.

However, other genetic studies seem to support the multi-regional or assimilation model, including the study by Hawks I mentioned earlier. He notes that traces of archaic humans in our DNA might not be easy to find, especially if hybrid children of H. erectus and H. sapiens intermarried with H. sapiens.

Though we may not know what happened during those many migrations out of Africa, one thing that's certain is that we evolved from an ancestor that we share in common with apes. She's often called a "common ancestor" because she's the creature whose children split into the two groups who eventually evolved into modern apes and modern humans. There is ample, persuasive evidence of human evolution taking place in Africa - evidence that comes from early human remains as well as genetics. So there is absolutely no question that H. sapiens had her origins in cultures and communities dramatically different from our own. So different that they belong to another species.

The big question is whether our ancestry is a "pure" lineage that springs from a line of H. sapiens who left Africa relatively recently, or a patchwork quilt of many peoples and cultures who intermingled as they spread across the globe. The answer, right now, is a matter of interpretation. Regardless of our origins, the H. sapiens of today seems to bear the cultural heritage of all three theories. We are a species of conquerers, assimilators, and mixed peoples who trade with each other across great distances.